Re: The Hunchback of the Mid-Cretaceous

["Jaime A. Headden" <qilongia@yahoo.com>]

Jura (archosaur@reptilis.net) wrote:

<On the archives I kept getting referenced to them being
strongly opisthoceolus. When did this end and the platycoelus
replacements start?>

  This goes against the fossils. *Spinosaurus*, as in
*Suchomimus* and *Baryonyx*, have strongly opisthocoelous
cervicals; mildly opisthocoelous cervicodorsals; anterior
dorsals with strong opisthocoely, but the anterior end is only
slightly ball-like, dorsally positioned, whereas ventrally it is
flat; and in posterior dorsals they become platycoelous, with a
flat cranial facet, but a mildly concave caudal facet on the
centrum. The sacrals have the same morphology as the dorsal
neural spines with proximal neural spine "enlargement" where
there seems to be a craniocaudally expanded base of the shaft.
The caudal vert lacks this, but as the element has a hollow
centrum, it is unlikely to pertain to an ornithopod (Read
Stromer, 1915 for more -- all! -- of the details, it's on the
Polyglot Paleontologist Site in English).

  *Spinosaurus* has the following autapomorphies:

  dorsal vertebrae lack well-defined spine-based lamina
extending to the zygapophyses (pre- and post-
spinozygapophyses); dorsal neural spines well over 6 times the
height of the corresponding centra in verts that the association
of centrum and neural arch are definite (some of Stromer's
positions are not well made, and I can relocate one neural arch
to another centrum and get a nearly perfect fit, based on the
illustrations in the paper, but that's that); dorsal ribs
strongly arched, making the trunk barrel shaped in section;
dentary expanded at the symphysis ventrally as well as dorsally
(unknown in *Irritator*); teeth bear strong lingual grooves
(*Irritator* has them all around the circumference of the
crown); splenial apparently fused? to the dentary and
supradentary (intercoronoid?); caudal neural spine caudally
deflected at a sharp angle, extremely elongated; cervicodorsal
and dorsal centra with a cranial and caudal ovate aspect of the
facets; dorsal and sacral neural spines expanded craniocaudally
at the base; posterior dorsals with reduced
postcentrodiapophyseal and diaparapophyseal laminae, nearly
absent in most-platycoelous verts.

  Most of these elements can be determined to belong to the same
general theropodan morphology, as avetheropod but not much more
finer than that. Stromer (1915) suggested the caudal may not
belong because it is too large comparatively to the sacral
centra preserved (2 1/2) and actually has a height greater than
that of the sacrals. It should be noted that, as Stromer (1915)
perceived, the elements have undergone a great deal of
mediolateral compression and distortion. The form of the centra
and hieght may be artificial taphonomically (i.e., their shape
is diagenic to the environment) but the know elements are well
founded. Should the animal ever prove to be a composite, I would
argue that the dorsal verts (which are homologous to one another
and form a definite series) serve as the type. The dentary is
certainly spinosaurid (see Sereno et al., 1998), but the verts
are highly reminiscent of *Baryonyx* albeit much shorter, in
being sub-rhomic in profile (they are restricted at the base,
narrow, with a distal expansion that tapers again at the top,
seen in most *Spinosaurus* vertebrae; anterior dorsals seem to
have a form more similar to *Suchomimus* in lacking the proximal
taper).

<And how likely is the hypothesis that _Spinosaurus_ could open
its sail like a Japanese fan (though not *half* as flexible as
that)?>

  This would have been very difficult unless one were a
dino-chiropractic: only the anteriormost dorsals would have
permitted this form of motion intravertebrally, so only the
anterior dorsals would have likely radially expanded, and not bt
much. It may have been more a function of a radiator expanding
surface. This does not mean the rest of the dorsal series could
not expand in another way: the dorsal ball and ventral flat
surface of the mid- to posterior dorsals would have allow some
marginal rotation on the long axis of the centrum, allowing each
spine to move lateral to the one next to it; this would have
expanded the tissue between each spine and permitted greater
surface area, but only by a marginal account.

  The ridiculous and TV-publicized scene in Jurassic Park 3
where the animal's sail is coming out of the water Jaws-like
will no dobut fire up yet more "Spinosaurs were Swimmers"
debate. Trust me, for the sail to have operated as a
hydrodynamic structure, spinosaurs would have to be able to gain
at least 30+ knots in open water: it acts as a stabilizer in
high speeds for such sail-finned fish as tuna, sailfish, and
marlin. The spine was incapable of sculling, and the caudals
were amphiplatyan, and unlikely to have allowed any sculling
ability as well. It could swim, I'm sure, but I doubt (really
doubt) that based on its, anatomy, it swam to fish. It would
need more than 4 fathoms of water to do this properly and be
able to hunt without creating large enough water quakes or waves
to alert the prey. The laterally compressed skull would have
been a hinderence in the water if the head was submerged.

  That's my peice on the Swimming Spinosaurus, Chimaeric
Spinosaurus, and Verts of Spinosaurus discussions.

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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